Pascal (Lazarus/Delphi)

curl with Variable Substitution in a JSON Request Body

See more CURL Examples

This example shows how to use variables inside a JSON request body using the {{variable_name}} syntax. When the HTTP request’s Content-Type indicates JSON, Chilkat automatically applies proper JSON escaping to each substituted value, ensuring the resulting JSON remains valid. Pascal (Lazarus/Delphi)

program ChilkatDemo;

// Demonstrates using the Chilkat Pascal wrapper via the C bridge DLL.
// Builds as a console application under Lazarus (FPC) or Delphi.

{$IFDEF FPC}
  {$MODE DELPHI}
{$ENDIF}
{$APPTYPE CONSOLE}

uses
  {$IFDEF UNIX}
  cthreads,
  {$ENDIF}
  SysUtils,
  CkDllLoader,
  Chilkat.HttpCurl,
  Chilkat.StringBuilder;

// ---------------------------------------------------------------------------

procedure RunDemo;
var
  success: Boolean;
  sbCurl: TStringBuilder;
  curl: THttpCurl;
  sbRawRequest: TStringBuilder;

begin
  success := False;

  //  Variable names are enclosed between {{ and }}
  //  Important: The variable {{var_name}} should be placed inside the quotes.

  //  This is correct:
  //  curl -X POST https://api.example.com/messages \
  //    -H "Content-Type: application/json" \
  //    -d '{"text":"{{message}}"}'

  //  This is incorrect:
  //    -d '{"text":{{message}}}'

  sbCurl := TStringBuilder.Create;
  sbCurl.AppendLn('curl -X POST https://api.example.com/messages \");
  sbCurl.AppendLn('  -H "Content-Type: application/json" \");
  sbCurl.AppendLn('  -d ''{"text":"{{message}}"}''');

  curl := THttpCurl.Create;

  //  In this example, the value we'll provide for the "message" variable
  //  will contain chars that require JSON escaping.
  curl.SetVar('message','He said "Hello, world!"');

  //  To demonstrate how the variables are replaced, this example does not execute the curl command. 
  //  Instead, it generates the raw HTTP request that would be sent if the curl command were run.
  sbRawRequest := TStringBuilder.Create;
  success := curl.ToRawRequest(sbCurl.GetAsString(),sbRawRequest);
  if (success = False) then
    begin
      WriteLn(curl.LastErrorText);
      Exit;
    end;

  WriteLn(sbRawRequest.GetAsString());

  //  The output is shown below.
  //  Notice that the quote chars around "Hello World!" are properly JSON escaped.

  //  POST /messages HTTP/1.1
  //  Host: api.example.com
  //  Content-Type: application/json
  //  Content-Length: 36
  //  
  //  {"text":"He said \"Hello, world!\""}


  sbCurl.Free;
  curl.Free;
  sbRawRequest.Free;

end;

// ---------------------------------------------------------------------------

begin

  try
    RunDemo;
  except
    on E: Exception do
      WriteLn('Unhandled exception: ', E.ClassName, ': ', E.Message);
  end;

  WriteLn;
  {$IFDEF MSWINDOWS}
  WriteLn('Press Enter to exit...');
  ReadLn;
  {$ENDIF}
end.